Engine and Airframe Designs Which Avoid FOD
Some military aircraft had a unique design to prevent FOD from damaging the engine. The design consisted of an S-shaped bend in the airflow, so that air entered the inlet, was bent back towards the front of the plane, and bent back again towards the back before entering the engine. At the back of the first bend a strong spring held a door shut. Any foreign object flying in the intake flew in, hit the door, opened it, flew through, and then exited the aircraft. Thus, only small objects swept up by the air could enter the engine. This design did indeed prevent FOD problems, but the constriction and drag induced by the bending of the airflow reduced the engine's effective power, and thus the design was not repeated.
The Russian MiG-29 fighter has a special engine design to prevent ingestion of FOD during take-off from rough airfields. The front air intakes could be closed and special inlets on the top of the plane temporarily opened. This would allow enough airflow to the engine for take-off but reduced the chances of the engine sucking up objects from the ground.
Another interesting design to minimize the risk of FOD is the Antonov An-74 which has a very high placement of the engines.
Boeing offered a gravel runway kit for early 737s that allows the plane to be used from unimproved and gravel runways. This kit included gravel deflectors on the landing gear; foldaway lights on the bottom of the plane; and screens that prevented gravel, entering the open wheelwells when the gear was extended, from hitting critical components. It also included vortex dissipators, devices that would reduce the airflow into the engine from the bottom so as to reduce the likelihood of ingesting gravel.
Airbus are investigating a novel approach to reducing FOD. By developing, in conjunction with Ricardo plc, the "Taxibot", a tractor controlled by the pilot, aircraft will not need to use jet engines while taxiing, so will not be vulnerable to FOD on aprons or taxiways.
Read more about this topic: Foreign Object Damage
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